1. Field of the Invention
This invention relates to selective permeable membranes, such as are used in reverse osmosis and ultrafiltration, having the ability to selectively separate a solvent from liquid mixtures such as solutions, emulsions and suspensions and a process for preparing the same.
2. Description of the Prior Art
Typical examples of a selective permeable membranes which are only permeable to a specific component of a solution or emulsion are reverse osmosis membranes and ultrafiltration membranes. The reverse osmosis membranes capable of separating a solvent from a solution containing substances of a relatively small particle size or substances of a low molecular size such as sodium chloride are useful in treating waste water from plants, purifying sewage and in the desalination of sea water and brine.
The ultrafiltration membranes, on the other hand, are capable of separating the solvent or dispersion medium from a solution or emulsion containing substances of a relatively large molecular or particle size such as colloids, proteins or microorganisms, and polymers and are used in the purification and concentration steps involved in the manufacture of foods and medicines as well as in the brewing and fermentation industries.
Heretofore, selective permeable membranes of this type have been made from cellulose acetates, polyamides, polysulfones, etc.
However, the selective permeable membranes made from cellulose acetates and polyamides exhibit not only low heat resistance but also poor chemical resistance, such as to strong alkaline or acidic substances making up the liquid mixtures treated by the membrane.
The permeable membranes made from polysulfones are more heat resistant and chemical resistant than those made from cellulose and polyamide but they are poorly resistant to organic solvents.
An attempt has recently been made to make a selective permeable membrane using an aromatic polyimide composed of an aromatic tetracarboxylic acid and diamine, but the manufacture requires converting the amide acid moiety in the molecular skeleton of the resulting selective permeable membrane to an imide ring at a high temperature and this additional step gives a product of low permeability.